(1) Field of The Invention
The present invention relates to an optical recording medium having a recording layer made of phase-change type recording material, and to a recording/erasing method therefor.
(2) Related art of The Invention
There have been recently remarked optical recording mediums which can be recorded with high density, and in which the recorded information can be erased and rewritten. In a phase-change type optical recording medium among these overwriteable optical recording mediums, the crystal state of the recording layer is changed by irradiating a laser beam thereto, and there is detected that change of reflectance of the recording layer, which is caused by such a change of state. The phase-change type of optical recording medium can be overwritten by a single light beam, so that it is superior to a magneto-optical recording medium, in that the optical system of driving apparatus for the former is simpler than that for the latter.
As material of a recording layer of the phase-change type optical recording medium, there are often adopted Ge--Te type materials which have a large difference of reflectance between crystal state and amorphous state, and have a relatively large stability in an amorphous state. The representative examples of such materials include chalcogen-type alloy materials such as Ge--Te, Ge--Te--Sb--S, Ge--Te--S, Ge--Se--S, Ge--Se--Sb, Ge--As--Se, In--Te, Se--Te, Se--As, as disclosed in U.S. Pat. No. 3,530,441.
Further, there have been proposed: the Ge--Te alloy materials such as added with Au (Japanese Unexamined Patent Publication No.61-219692), Sn and Au (Japanese Unexamined Patent Publication No.61-270190), Pd (Japanese Unexamined Patent Publication No.62-19490) aiming at improving the stability and high crystallization speed; and the Ge--Te--Se--Sb material with a specified composition ratio aiming at improving the recording/erasing repeatability characteristics (Japanese Unexamined Patent Publication No.62-73438).
However, none of these materials completely satisfies all of the characteristics required in a recording layer of a phase-change type overwriteable optical recording medium. Particularly, it has become the most important object to improve the recording sensitivity and erasing sensitivity, to avoid deterioration of erase ratio due to erase failure at overwriting, and to improve the life span of recording/erasing repeatability of both the recorded and unrecorded portions.
In view of the above, it has been recently proposed to apply a compound called "chalcopyrite". The chalcopyrite type compounds have been widely investigated as compound type semiconductor materials, and applied such as to solar cell. The chalcopyrite type compounds have a composition represented by I b-III b-IVb.sub.2 or II b-IVb-Vb.sub.2 in terms of the chemical periodic table, and have a structure comprising layered two diamond-structures.
It is known that, among these chalcopyrite type compounds, AgInTe.sub.2 can be particularly used as a material of recording layer of optical recording medium, by diluting it such as with Sb and Bi (Japanese Unexamined Patent Publication Nos. 3-240590, 3-99884, 3-82593, and 3-73384).
In addition to such phase-change type optical recording mediums adopting chalcopyrite type compounds, there are disclosed other phase-change type optical recording mediums in which a AgSbTe.sub.2 phase is created upon crystallization of recording layer, such as in Japanese Unexamined Patent Publication Nos. 4-267192, 4-232779, and 6-166268. The optical recording mediums adopting these recording materials have an improved C/N, erase ratio, modulation factor, and recording sensitivity.
However, the recording layer of the chalcopyrite compound having the aforementioned composition has a life span of recording/erasing repeatability in the order of several thousand times at the utmost. Thus, there remains such a problem that the chalcopyrite type compound is not necessarily complete, viewing the fact that the life span of recording/erasing repeatability of magneto-optical disks is hundred thousand times.